Imaging and processing for plasma ion source

ABSTRACT

Applicants have found that energetic neutral particles created by a charged exchange interaction between high energy ions and neutral gas molecules reach the sample in a ion beam system using a plasma source. The energetic neutral create secondary electrons away from the beam impact point. Methods to solve the problem include differentially pumped chambers below the plasma source to reduce the opportunity for the ions to interact with gas.

This Application claims priority from U.S. Provisional Application 61/653,930, filed May 31, 2012, which is hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to focused ion beam systems that use a plasma ion source and that can form a sub-micron spot on the work piece.

BACKGROUND OF THE INVENTION

Focused ion beam systems are used to fabricate or alter microscopic or nanoscopic structures. Some focused ion beam (FIB) columns use a plasma ion source, such as the inductively coupled plasma (ICP) source described in U.S. Pat. No. 7,241,361, which is assigned to the assignee of the present invention. Focused ion beam systems using plasma ion sources, such as that described above, have several advantages over systems using liquid metal ion sources. For example, plasma ion sources can provide a higher brightness with reduced energy spread. Plasma ion sources can provide a variety of ion species, some of which have higher sputter yields and do not contaminate the sample.

FIG. 1 shows schematically an ICP system according to the prior art that is described in U.S. Pat. No. 7,241,361. System 100 includes an ion beam column 102 with a plasma ion source including a plasma chamber 104 which is supplied with gas or gases through a capillary or flow restrictor through gas inlet 106. A coil 107 is coupled by an impedance matching circuit to an RF source, not shown, to supply energy to ionize the gas in the plasma chamber 104. System 100 preferably includes a means to reduce the energy spread of ions in the ion beam. Such a means can include a split Faraday shield to reduce capacitive coupling between the an105tenna and the plasma or a balanced antenna. Ionized gas atoms or molecules are extracted from plasma chamber 104 by an extraction electrode 105 that pulls ions through an aperture 108 that serves as a source electrode that electrically biases to the plasma to a high potential, that is, greater than 10,000 V. Ions are accelerated toward a work piece 110 positioned on an adjustable stage 112. Ion beam systems also typically include a beam blanker 130 for blanking the ion beam, beam deflectors 132 for positioning the beam, and a focusing lens 134 to collimate or focus the beam of ionized molecules.

Typically, only a very small percent of the atoms or molecules in the plasma chamber are ionized. Neutral atom can diffuse through the aperture from which the ions are extracted. The neutral atoms, however, have very low energy because they are not accelerated by the extraction electrode. It has been thought that very few of the low energy neutral atoms reach the work piece, because they diffuse from the aperture in random directions, collide with elements of the optical column, and are removed by the vacuum pump.

Focused ion beam systems are also used to form microscopic structures by etching material from a work piece or depositing material onto a work piece. The ions can remove material from the surface by sputtering, that is, momentum transfer from the ion to atoms in the work piece. The ions can also activate a precursor gas that decomposes in the presence of the ion beam to deposit a material or to form a volatile compound with the target material to enhance etching of the target. Ion beam systems often also include a gas inlet for injecting precursor gases 140 and a needle 142 for directing the flow of precursor gases toward the work piece surface.

Focused ion beam system also typically include a secondary electron detector 150, such as an Everhart-Thornley detector, for forming an image as the ion beam scans the sample surface and the ion beam impact produces secondary electrons. The image contrast at each point is determined by the number of secondary ions detected. Applicants have found that secondary electron images formed by the ion beam from a plasma source can have unexpectedly poor contrast. An improved apparatus and method to collect higher quality secondary electron images and to produce higher resolution etching or deposition is therefore needed.

SUMMARY OF THE INVENTION

An object of the invention is to improve processing from a focused ion beam system using a plasma ion source.

Applicants have found that collisions between energetic ions and neutral gas produces high kinetic energetic neutral particles that impact the work piece, thereby degrading imaging and processing. By reducing the contact of the energetic ions with gas diffusing from the plasma chamber, the number of high kinetic energy neutral ions reaching the sample surface is reduced, improving ion beam imaging and processing. In one embodiment, a series of differentially pumped chambers reduce the number of collisions between energetic ions and neutral gas by reducing the amount of gas encountered by the ion beam, thereby reducing the creation of high kinetic energy neutral particles impacting the work piece.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a schematic of a prior art ion beam system having a plasma source.

FIG. 2 shows a schematic of a four chamber embodiment of the present invention.

FIG. 3 shows a schematic of another embodiment of the invention.

FIG. 4 is a flow chart of an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Ions exit the plasma chamber at high kinetic energies, accelerated by the extraction electrode and cathode. The ions are formed into a beam and focused in the focusing column to a fine spot on the work piece. The beam is typically scanned across the sample surface using electrostatic deflectors. Neutral atoms also escape through the aperture, but most of the low speed neutral gas particles collide with elements in the column and very few were thought to reach the work piece.

By investigating the cause of poor contrast in secondary electron images created using a plasma ion source, applicants have found that poor contrast is caused by “energetic neutrals.” “Energetic neutrals” are created when an ion with appreciable energy interacts with gas molecules in the beam path in such a manner that a charge exchange takes place. At least some of these “energetic neutrals” have a minimal trajectory change from that of the original ion and because they cannot be focused or deflected travel in a straight line to the sample surface.

The impact of the energetic neutrals with the sample releases secondary electrons, but because the neutral atoms are not focused or scanned with the ion beam, the secondary electron current is emitted from a wide area of the sample. The secondary electron current from the energetic neutrals thus constitutes a “noise” that is detected on top of the secondary electron signal from the beam impact point. The noise reduces the contrast of the ion beam image and can be sufficiently strong to wash out the signal. The energetic neutral particles also cause sputtering and, if a precursor gas is present in the chamber, decomposition of the precursor gas to cause etching or deposition away from the ion beam impact point.

Focused ion beam system using a plasma ion source were typically not used for high precision fabrication because the high energy spread of ions from a plasma source plasma made it difficult to form a small spot. A plasma ion source, as described for example in U.S. Pat. No. 8,053,725, can provide an ion beam in which the ions have a low energy spread. The plasma ion source is more versatile than the liquid metal ion source and can produce a high beam current or a reduced beam current with higher precision. For example, a plasma ion source can produce a beam having an energy spread of less than about 10 eV, and a spot size of less than about 25 nm at a current of about 2 pA. With the development of a high resolution FIB using a plasma ion source, applicants have produced a system that is being used in application previously reserved for liquid metal ion sources. Because of the high resolution of the FIB with the plasma ion source makes possible new applications, new problems arise that were not previously encountered. The problem of reduced image resolution is one such problem.

In the system of FIG. 1, some of the neutral particles leaking from the plasma source through aperture 108 can pass through to ion column 102. The gas pressure in a plasma chamber typically varies between about 10⁻³ mbar and about 1 mbar, providing ample opportunity for collisions between ions extracted from the ion source and neutral particles thus creating energetic neutrals directed along the ion column.

Ions extracted by the extraction electrode 105 and accelerated by the potential difference between the plasma and the focusing column collide with those neutral particles and some are neutralized, but retain most of their energy and their momentum toward the work piece. Those energetic neutral particles reach the work piece 110. Because neutral particles do not respond to the fields of the focusing lenses 134, neutral particles that are not blocked by the aperture plates 120 are spread out over areas of the work piece surface 110, typically this is a much larger area than the area scanned by the focused ion beam. While it had been previously thought that very few neutral atoms reach the specimen surface, it appears that the energy neutral particles created by the neutralization of the energetic ions do reach the surface in sufficient quantities to produce adverse effects.

FIG. 2 shows a schematic of an embodiment of the present invention including a focused ion beam column 200 using a plasma chamber 201. Elements of the plasma ion source and the ion column are not shown but are similar to those shown in FIG. 1. A high pressure gas source 202 is connected to plasma chamber 201 via a flow restrictor 204, such as a capillary tube or needle valve, that restricts the flow of gas into plasma chamber 201 and drops the pressure in plasma chamber 201 to typically between 10⁻³ mbar and 1 mbar. The term flow restrictor 204 may be interchangeable with the term aperture which equivalently restricts flow of a gas into the plasma chamber. Ions leave plasma chamber 201 through an exit aperture 205, shown schematically as a tube in FIG. 2. Exit aperture 205 has a diameter sufficiently large enough to accommodate the required ion current and in some embodiments the diameter can be close to 2 mm. In some embodiments, the exit aperture is electrically biased to a high voltage to bias the plasma.

Ions leave plasma chamber 201 through the exit aperture 205 and pass through to a first intermediary evacuated vacuum chamber 206. Neutral atoms also diffuse out of exit aperture 205 and into first intermediary evacuated vacuum chamber 206. Collision between the energetic ions having a great momentum in a direction toward the specimen and the less energetic diffusing neutral gas causes a charge exchange that results in energetic neutral particles having a great momentum in the direction of the specimen. By reducing the total amount of gas between the plasma chamber and the specimen, the number of collisions is reduced and fewer energetic neutrals are created.

A measure of the probability of creating energetic neutrals is the product of the number of collisions between energetic ions and neutral gas particles and the probability of the energetic ion picking up a charge from the neutral gas. The probability of a collision depends on the gas pressure in the region through which the beam passes and the path length through that region. Embodiments of the invention reduce the sum of products of pressure and path length by dropping the pressure rapidly to shorten the path length through the higher pressure regions. Embodiments of the invention provide in the beam path multiple differentially pumped chambers separated by pressure limiting apertures, to reduce the gas pressure as the beam gets further from the plasma chamber. While ideally the first vacuum chamber below the plasma would be pumped down to the desired pressure to remove all the gas, in practice, multiple chambers are required because of the practical limits on the pumping speed and the diffusion of gas through the apertures. Further, it is very difficult to transition from a region of high pressure to one of low pressure in which there is an aperture of significant size.

In a preferred embodiment, the sum of the products of the pressure time the path length is less than 3E-3 mbar*mm, preferably less than 3E-4 mbar*mm, and most preferably less than 3E-5 mbar*mm. The probability of creating an energetic neutral is a complicated function of many factors, but for some reasonable assumptions, for each ion launched down the column the pressure length products achieved as described thus preferably leads to a probability of neutral creation that is reduced to less than 1%, more preferably to less than 0.1%, and most preferably to less than 0.01%. The probability of an energetic neutral particle reaching the specimen depends on the probability of the collision that produced the energy neutral not producing such a deflection. This probability also depends on the energy of the energetic ions. Multiple collisions can also play a role in determining how many energetic neutrals reach the specimen. A further complication is that for each combination of incident ion and target neutral gas species a different probability will result.

Intermediary vacuum chamber 206 is pumped by a high capacity pump 208 capable of handling noble gases, which could be a turbo pump, and is capable of maintaining the chamber 206 at about 10⁻⁵ mbar to 10⁻⁶ mbar. The ion beam and some neutral gas molecules then pass from vacuum chamber 206 through a differential pumping aperture (DPA) 209 to a second intermediary evacuated vacuum chamber 210. The length of the aperture is preferably greater than twice the diameter so that a sufficient vacuum differential can be achieved between the first intermediary evacuated vacuum chamber 206 and the second intermediary evacuated vacuum chamber 210.

Vacuum chamber 210 is evacuated using a second turbo pump 212, which maintains a vacuum of preferably less than 10⁻⁶ mbar more preferably less than 10⁻⁷ mbar. The ion beam and very few neutral gas molecules pass from intermediary vacuum chamber 210 through a second DPA 213 to a final evacuated vacuum chamber 214 which is evacuated using an ion pump 216, which is capable of achieving a very high quality vacuum preferably in the range of 1E-8 mbar to 5E-7 mbar. DPA 213 is typically a tube that is one or two millimeters in diameter and a few millimeters in length.

Intermediary evacuated vacuum chamber 214 is the longest of the intermediary vacuum chambers allowing the beam to spend the most time in the highest quality vacuum environment further reducing the opportunity for energetic neutrals to form. The ion gas beam leaves vacuum chamber 214 through a final DPA 217, typically with a diameter of one or two milliments, into a sample chamber 218. An evacuation system such as a high powered turbo pump 220 maintains a vacuum of preferably less than 1*10⁻⁵ mbar more preferably less than 1*10⁻⁶ mbar, and even more preferably less than or equal to about 5*10⁻⁷ mbar in the sample vacuum chamber 218 during processing.

The pressure in the sample chamber 218 is typically greater than the pressure in the previous chamber 214. The pressure in the sample chamber 218 may be raised by the use of a precursor gas, or desorption of gases adsorbed onto surfaces when the sample chamber is opened to insert or remove a work piece. A valve may be included to seal chamber 214 when sample chamber 218 is opened to the atmosphere to insert a work piece.

According to one embodiment of the present invention, the plasma gas source 202 is set at a relative pressure (P₀). Preferably, the pressure (P₁) of the plasma chamber 201 is less than P₀. Even more preferably, the pressure (P₂) of intermediary vacuum chamber 206 is less than P₁, and the pressure (P₃) of intermediary chamber is less than P2, In certain embodiments, the pressure (P₄) of intermediary chamber 214 can be either greater or less than P₃ depending on processing conditions. Finally, the pressure of sample chamber 218 is preferably either greater or less than P₄ based on processing conditions.

Those skilled in the art will readily realize that components of the ion beam column such as focusing lenses, beam deflectors, and beam blankers can be placed at any of the different intermediary chambers. Preferably, the final focusing lenses are placed toward the bottom of chamber 214 because optimal spot size is achieved when the final lens to sample distance is minimized.

FIG. 3 shows a schematic of an embodiment of an inductively coupled plasma (ICP) ion beam column 300 according to the method of the present invention. In this system, two evacuated vacuum chambers 306 and 308 are employed in series rather than the three described above. Ions leave plasma chamber 201 through an exit aperture 314, shown schematically as a tube in FIG. 2. Exit aperture 314 has a diameter sufficiently large enough to accommodate the required ion current and in some embodiments the diameter can be close to 2 mm. In some embodiments, the exit aperture is electrically biased to a high voltage to bias the plasma. Ions pass through to a first intermediary evacuated vacuum chamber 306. Neutral atoms also diffuse out of exit aperture 314 and into first intermediary evacuated vacuum chamber 306.

Intermediary vacuum chamber 306 is attached to a pump 322, which is preferably a high capacity pump capable of handling noble gases and maintaining the chamber 306 at about 10⁻⁵ mbar to 10⁻⁶ mbar. The ion beam and some neutral gas molecules then pass from vacuum chamber 306 through a DPA 316 to a second intermediary evacuated vacuum chamber 308 which is evacuated using an ion pump 324. Chamber 308 is the longest of the intermediary vacuum chambers allowing the beam to spend the most time in the highest quality vacuum environment further reducing the opportunity for energetic neutrals to form. The ion gas beam leaves vacuum chamber 308 through a final DPA 318 into a sample chamber 218. It is noted that the term differential pumping aperture is used throughout the description above, but is not meant to limit the present invention. The term DPA is interchangeable with many types of apertures such as a conductance limiting aperture or a pressure limiting aperture.

It is further noted that in the embodiment shown in FIG. 3, the set of intermediary vacuum chambers 306 and 308 and their respective exit apertures and pumps can be identical to any combination of two of the three chambers shown in FIG. 2. Preferably, the pressure at the plasma gas source 202 is highest and the pressure drops as the ions travel through each chamber. In certain embodiments, the pressure at sample chamber 218 is greater than the pressure at intermediary chamber 308 depending on processing conditions.

FIG. 4 is a flow chart of an embodiment of improving ion beam processing in a focused ion beam column using a plasma ion source. In step 402 energy is applied to a gas in the plasma chamber ionizing some of the gas molecules to produce a plasma. Because the gas supplied to the plasma chamber is typically at a very high pressure, the plasma chamber typically has a pressure of between 10⁻³ mbar to 1 mbar. In step 404 ionized atoms or molecules are extracted from the plasma chamber using an extraction electrode through an aperture into a first vacuum chamber. At this point some neutral atoms also diffuse through the extraction aperture into the first vacuum chamber. These neutral atoms have very low energy because they are not accelerated by the extraction electrode. It is believed that these low energy neutrals diffuse within the vacuum chamber in random direction and do not continue along the ion column produced by the extraction electrode.

The first vacuum chamber is pumped to reduce the pressure to between 10⁻⁵ mbar and 10⁻⁶ mbar. By reducing the pressure in the vacuum chamber there are fewer neutral gas molecules the ion beam needs to pass through thus reducing the likelihood of collisions between ionized atoms and neutral molecules. The first vacuum chamber is configured to be preferably as short as possible further reducing the opportunity for the ions to collide with neutral molecules before passing through a pressure limiting aperture into second vacuum chamber in step 406. The pressure limiting aperture makes it possible to decrease the pressure in the second chamber even further, typically in the range of 5*10⁻⁷ mbar to 1*10⁻⁷ mbar if using a turbo pump and even further, for example 5*10⁻⁸ if an ion pump is employed for evacuation.

If more than two vacuum chambers are used, conditional step 408 repeats step 406 with a further reduction in the pressure in each subsequent vacuum chamber with the objective being to get the pressure to as low as possible and reduce the number neutral molecule available for collisions that create energetic neutrals. The final vacuum chamber typically has the lowest pressure and is typically longer than the previous vacuum chambers to allow the ion beam to spend the longest time in the lowest pressure environment where collisions with neutral molecules are the least likely.

In step 410 the beam is passed through a pressure limiting aperture into a sample chamber where it is then used to process a work piece in step 412. The pressure limiting aperture between the final vacuum chamber and the sample chamber makes it possible to have a higher pressure in the sample chamber, which would occur is a precursor gas were being used, while still maintaining a very low pressure in the final vacuum chamber. It is noted that the ions can be focused in any of the intermediary vacuum chambers as described above, but the final probe forming focusing is preferably done at the final intermediary vacuum chamber before reaching the sample chamber, but as close to the sample as is possible.

Once processing of the work piece is completed the method is complete.

Embodiments of the present invention can provide one or more advantages over typical prior art ICP ion beam systems. The primary advantage of all of the embodiments is reducing the number of neutrals created as the ion beam passes along the ion beam column between the ion extraction region and the sample surface.

It will be appreciated by those of ordinary skill in the art of ICP ion technology that the multiple vacuum chamber system described above makes it possible to achieve a very high vacuum within a minimal distance travelled from the ion source and that a very high vacuum environment reduces the number of “energetic neutrals” reaching the work piece by reducing the opportunity for ion collisions with neutral gas molecules as the ion beam travels down the column.

In certain embodiments, the multiple vacuum chamber described above can be used in dual-beam systems such as Scanning Electron Microscope (SEM)/ICP-FIB systems. In such a configuration, it is particularly desirable to mitigate neutral particle production because the secondary electrons generated by the FIB can harm SEM imaging.

It will be appreciated by those of skill in the art that the invention described is not limited to inductively coupled plasma sources. The invention described above can be used with other plasma sources, such as a plasma source based on electron cyclotron resonance (ECR).

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

We claim as follows:
 1. A charged particle beam system comprising: a plasma chamber for containing a plasma; a source electrode for biasing the plasma to a voltage of at least 10,000 V; an extraction electrode for extracting ions from the plasma chamber; a focusing lens for focusing the ions into a beam directed toward the work piece; a sample chamber for containing a work piece, the sample chamber connected to a vacuum pump; and a first intermediary vacuum chamber connected at one end to the plasma chamber and at the other end through a differential pumping aperture to the sample chamber or to one or more additional intermediary vacuum chamber, the first intermediary vacuum chamber connected to a vacuum pump, the first and the one or more additional intermediary vacuum chamber or chambers reducing the collision of the ion beam with neutral gas particles, thereby reducing the creation of energetic neutral particles that impact the work piece.
 2. The charged particle beam system of claim 1 further comprising at least one additional intermediary vacuum chamber between the first intermediary vacuum chamber and the sample chamber, each of the at least one additional intermediary vacuum chambers connected to a vacuum pump and separated from the preceding and succeeding chambers by differential pumping apertures.
 3. The charged particle beam system of claim 2 in which each of the additional intermediary vacuum chamber has a lower pressure than the immediately preceding intermediary vacuum chamber.
 4. The charged particle beam system of claim 3 in which each of the additional intermediary vacuum chamber has a lower pressure less than 1/10 the pressure of the immediately preceding intermediary vacuum chamber.
 5. The charged particle beam system of claim 1 in which the first intermediary vacuum chamber has a pressure at least five times lower than the pressure in the plasma chamber.
 6. The charged particle beam system of claim 1 in which the differential pumping aperture leading from the first intermediary vacuum chamber is at least twice as long as its diameter.
 7. The charged particle beam system of claim 1 in which the sample chamber has a pressure higher than the immediately preceding intermediary vacuum chamber.
 8. The charged particle beam system of claim 1 in which the sum of the products of the pressure times the path length through each of the intermediary vacuum chambers and the sample chamber is less than 3×10⁻³ mbar*mm.
 9. The charged particle beam system of claim 8 in which the sum of the products of the pressure times the path length through each of the intermediary vacuum chambers and the sample chamber is less than 3×10⁻⁵ mbar*mm.
 10. The charged particle beam system of claim 1 in which the probability of energetic neutral creation is less than 1%.
 11. The charged particle beam system of claim 1 in which the probability of energetic neutral creation is less than 0.01%.
 12. The charged particle beam system of claim 1 further comprising means to reduce the energy spread of ions emitted from the plasma chamber to less than 10 eV.
 13. A method of improving ion beam processing using a plasma ion source by reducing the number of energetic neutral particles impacting onto the work piece, comprising: applying energy to a gas in a plasma chamber to produce a plasma, the plasma chamber being maintained at a first pressure; extracting ions from the plasma chamber into a first intermediary vacuum chamber, the first intermediary vacuum chamber being maintained at a pressure lower than the pressure in the plasma chamber; passing the ions into a sample chamber through a differential pumping aperture; and focusing the ions onto a work piece to process the work piece with the focused ions.
 14. The method of claim 13 further comprising passing the ions through one or more additional intermediary vacuum chambers, the pressure in each of the one or more additional intermediary vacuum chambers being lower than the pressure in the previous intermediary vacuum chamber.
 15. The method of claim 13 in which the product of the pressure times the path length through each of the intermediary vacuum chambers and the sample chamber is less than 3×10⁻³ mbar*mm.
 16. The method of claim 13 in which the product of the pressure times the path length through each of the intermediary vacuum chambers and the sample chamber is less than 3×10⁻⁵ mbar*mm.
 17. The method of claim 13 in which the probability of energetic neutral creation from the first intermediary vacuum chamber to the work piece is less than 1%.
 18. The method of claim 13 in which the probability of energetic neutral creation from the first intermediary vacuum chamber to the work piece is less than 0.01%
 19. The method of claim 14 in which the first intermediary vacuum chamber is maintained at a pressure of between 10⁻⁵ mbar and 10⁻⁶ mbar and the subsequent one of the additional intermediary vacuum chambers is maintained at a pressure of between 10⁻⁶ mbar and 10⁻⁷ mbar.
 20. The method of claim 13 in which extracting ions from the plasma chamber into a first intermediary vacuum chamber includes extracting ions having an energy spread of less than 10 eV. 